Sulfur hexafluoride is a nonflammable gas of low toxicity that is considered to be inert in most applications. It is used for plasma etching in semiconductor processing and as a gaseous insulator in high voltage electrical devices such as cables, capacitors and transformers. Traces of oxygen and moisture are undesirable in these applications.
Prior methods for purification of sulfur hexafluoride include cryogenic separations, as described, for example, in United Kingdom Patent 1,107,363. The process is energy intensive and, unlike the process of the present invention, is not well suited to point-of-use applications. The separation is based on differences in boiling points and is not capable of removing trace impurities to the part per million level.
Molecular sieves can be used to adsorb impurities such as water and carbon dioxide from sulfur hexafluoride as disclosed in U.S. Pat. No. 3,675,392. In the present invention impurities are irreversibly bound by chemical reaction. Molecular sieves require activation at high temperatures and, unlike the reaction products of the present invention, give no visual indication of when they are saturated. Activated carbon is useful for adsorbing impurities, such as carbon fluorides, formed in the manufacture of sulfur hexafluoride (see Japanese examined patent application JP 75-008040). It is not capable of removing oxygen and moisture.
Calcium hydride has been reported to react with water and carbon dioxide impurities in sulfur hexafluoride (see Japanese unexamined patent application JP 60-054723). Calcium hydride has a lower capacity for impurity removal than the product used in the present invention and is not reactive toward oxygen.